1. Field of the Invention
This invention relates to a driving apparatus for needles of a knitting machine, and more particularly to a driving apparatus for moving a needle by means of a linear motor.
2. Description of the Related Art
A flat-knitting machine wherein each knitting needle is moved by a thin linear motor of a flat plate-like shape is disclosed, for example, in Japanese Patent Application Publication No. 1-12855. A linear motor for a knitting machine of the type includes a flat plate-like stator assembly, a flat plate-like moving assembly connected to the needle, and a position sensor for detecting the position of the moving assembly with respect to the stator assembly.
The stator assembly and the moving assembly are arranged in parallel to each other so as to cooperatively form a flat plate-like linear motor. The moving assembly is held and guided at upper and lower portions thereof so as to assure a high degree of accuracy in movement thereof by a pair of guides mounted on the stator assembly and elongated in the direction of movement of the moving assembly, and by a pair of bearings mounted on the moving assembly and individually fitted with the guides so as to move in the direction of movement of the moving assembly.
A linear motor of the type described above is assembled, for example, in the following manner. The guides in pair are first assembled into the stator assembly, and the bearings in pair are assembled into the moving assembly. Then, the bearings in pair are assembled into the guides. Thereafter, an operation of adjusting the mounting positions and the mounting condition of the guides on the stator assembly and another operation of adjusting the mounting positions and the mounting condition of the bearings on the moving assembly are performed simultaneously so that the moving assembly may move smoothly relative to the stator assembly and the guides.
The mounting condition of the guides on the stator assembly such as the parallelism and the distance between the guides mounted on the stator assembly and the mounting condition of the bearings on the moving assembly such as the parallelism and the distance between the bearings mounted on the moving assembly have such a relationship that, if one of them is varied, the other must be also varied. Accordingly, the operations described above are complicated and the assembling operations of the guides and the bearings into the stator assembly and the moving assembly are very cumbersome. Therefore, much skill is required for an assembling operation of the linear motor.
Meanwhile, in a knitting machine, the higher the accuracy in position and speed of movement of needles are, the higher the quality of a knit fabric becomes. Therefore, it is desired for a linear motor for a knitting machine to assure enhanced accuracy in position control and speed control of a moving assembly with respect to a stator assembly to allow the moving assembly to move smoothly and accurately over the overall range of movement of it in its direction of movement.
However, in the conventional linear motor for a knitting machine described above, an exciting coil of the stator assembly is disposed on a thin plate member such as a metal plate. Further, magnetic forces generated from the coil arranged on the stator assembly and a permanent magnet arranged on the moving assembly mutually act. From the two reasons just described, the plate member of the stator assembly and a plate member of the moving assembly are deformed when assembling the guides and the bearings into the stator assembly and the moving assembly, respectively, when assembling the moving assembly into the stator assembly and when moving the moving assembly. As a result, the parallelism between the guides or the parallelism between the bearings becomes no longer accurate, making it less easy for the moving assembly to move, thereby disturbing smooth movement of the moving assembly.
Therefore, in the conventional linear motor for a knitting machine, the accuracy in holding and guiding the moving assembly by the pair of guides and bearing is set comparatively low in order to assure smooth reciprocating movement of the moving assembly. In other words, a large play is provided at a coupling portion between the stator assembly and the moving assembly to assure smooth reciprocating movement of the moving assembly.
However, where such a large play is provided at the coupling portion between the guide and the bearing for coupling the stator assembly and the moving assembly, a uniform distance is not assured between the stator assembly and the moving assembly. Consequently, the position of the moving assembly with respect to the stator assembly cannot be detected accurately. As a result, the position and the speed of movement of the moving assembly with respect to the stator assembly cannot be controlled accurately.